Bug 19977: Open only .pref files in Local Use tab (sysprefs)

Bug 19977: Open only .pref files in Local Use tab (sysprefs)

Test plan:
1. Open .../prog/en/modules/admin/preferences/circulation.pref
with vim (check that it creates a .circulation.pref.swp in the same
directory)
2. Go to the "Local Use" tab in admin/systempreferences.pl
3. Confirm that the page works normally

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>